1. Field of the Invention
The present invention relates to an angle of rotation sensor such as a steering angle sensor. More particularly, the present invention relates to an optoelectronic angle of rotation sensor having an illuminated code disk carrying a multi-track digital code and being coupled to the rotational movement of a rotor, and having a sensor array for scanning the multi-track digital code on the code disk. The sensor array having a plurality of individual transducer elements and being arranged transverse to the moving direction of the code disk with respect to the longitudinal direction of the sensor array.
2. Background Art
Angle of rotation sensors are frequently used for realizing automatic positioning and measuring processes in machine tools and coordinate measuring systems. In addition, angle of rotation sensors are used in the automotive industry for determining the absolute angular position of a steering wheel. Such angle of rotation sensors are referred to as steering angle sensors.
In motor vehicles, a value indicative of the steering angle is required for influencing a dynamic control system with this value. In addition to the steering angle information, a dynamic control system receives other measuring data, for example, the rotational speed of the wheels or the rotation of the motor vehicle about its vertical axis. The dynamic control system evaluates the absolute steering angle, as well as the steering speed, together with other measuring data in order to control actuators, such as brakes, and/or for engine management.
DE 40 22 837 A1 discloses an optoelectronic steering angle sensor having a light source and a line sensor arranged parallel to one another and spaced apart from one another. A code disk is arranged between the light source and the line sensor. The code disk is connected to the steering spindle in a rotationally rigid fashion. The line sensor is a charge coupled device (CCD) line sensor having a plurality of individual transducer elements. The code disk carries a code having a light slot that is realized in the form of an Archimedean spiral extending over 360xc2x0. Information on the actual steering angle can be obtained from the illumination of corresponding transducer elements of the line sensor at a certain steering deflection.
The Archimedean spiral used as the code has a continuous progression. This means that this code represents an analog code. In this angle of rotation sensor, only a fraction of the transducer elements of the line sensor participate in determining the angular position of the steering wheel. Namely, the transducer elements that are illuminated through the light slot of the analog code. The remaining transducer elements of the line sensor only participate indirectly in the evaluation of the angular information because the non-illuminated transducer elements merely make it possible to ascertain that the angular position of the steering wheel is not situated in the regions represented by these non-illuminated transducer elements.
When using such an analog code realized in the form of a light slot, it is possible for the light slot to become contaminated by debris such as hair. This means that the steering angle can no longer be determined in this code region because the code is sectionally shaded by the contamination. The transducer elements which need to be illuminated in order to determine this angular position remain dark. Consequently, this angular position is not defined.
In addition to the previously described analog code, the contamination problem also occurs with angle of rotation sensors having a digital code in the form of several tracks arranged adjacent to one another. If one or more tracks of the digital code are contaminated in an angular position of the rotor relative to the stator, then this angular position can not be determined with a desired resolution.
Accordingly, it is an object of the present invention to provide an optoelectronic angle of rotation sensor having a multi-track digital code operable to provide a reliable angle measurement in the event that the digital code is partially contaminated.
The present invention obtains this object and other objects by providing the individual code tracks of a multi-track digital code on the code disk several times with the corresponding individual tracks being spaced apart from one another across the width of digital code transversely with respect to the direction of movement of the code disk.
In the angle of rotation sensor according to the present invention which, in particular, is also suitable as a steering angle sensor for determining the absolute angular position of a steering wheel of a motor vehicle, at least individual code tracks are provided several times in the multi-track digital code of the code disk. Preferably, the code tracks having a particularly high significance with respect to angle-related information are provided several times. In order to lower the statistical probability that the code tracks which are contained in the digital code several times are simultaneously impaired with respect to their function by contaminations in the same position, these code tracks are spaced apart from one another across the digital code transversely with respect to the direction of movement of the code disk. Preferably, other code tracks of the digital code having a lower significance with respect to the angular information contained therein are arranged between the code tracks that are provided several times.
If the code disk of an optoelectronic angle of rotation sensor has a comparatively small width, the present invention proposes that the individual code tracks only have a minimal width. For example, the code tracks have a width such that respective code tracks only illuminate three transducer elements of a sensor array. In comparison, respective code tracks in accordance with the prior art usually have such a width that they illuminate six or more transducer elements. In accordance with the prior art, such a relatively larger width is sensible because it is assumed that a possible contamination does not cause the light to be completely blocked due to the relatively large width of the tracks.
In the angle of rotation sensor according to the present invention, the probability that a code track of the digital code, at least a highly significant code track, is not projected onto the sensor array at all due to a contamination in a certain angular position is substantially reduced because the two identical code tracks are spaced apart from one another. Instead of merely providing individual code tracks several times in the digital code of the code disk, it would also be possible to arrange, for example, all code tracks several times on the code disk twice. If all code tracks are provided several times, all multiple code tracks may be spaced apart from one another by the same distance in order to fulfill the requirement of spacing apart from code tracks that are provided several times. This corresponds to a parallel arrangement of the respective multi-track digital code. It is also possible for code tracks with the same angular information of higher significance to be spaced apart from one another by a larger distance than code tracks with a lower significance with reference to the angle-related information contained in the respective code track.
Because the present invention makes it possible to reduce the width of an individual code track, the code disk of the steering angle sensor according to the present invention does not have to be realized larger than in known systems. This means that a sensor array of customary dimensions, for example, a line sensor, can be used in the angle of rotation sensor according to the present invention.
Because at least individual code tracks are provided several times, it is also possible to place the multiple code tracks offset relative to one another, for example, by one-half bit width measured on the width of the bit with the lowest significance. The resolution of the angle of rotation sensor can be increased in this fashion. It is also possible for this offset to replace a code track such that the required width of the code disk is correspondingly reduced.